In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
If a press is to print In more than one color, a separate printing plate corresponding to each color is required. Each set of cylinders associated with a particular color on a press is usually referred to as a printing station. Each such station typically includes an impression cylinder, a blanket cylinder, a plate cylinder and the necessary ink (and, in wet systems, dampening) assemblies. The recording material is transferred among the print stations sequentially, each station applying a different ink color to the material to produce a composite multicolor image.
The number of print stations on a press depends on the type of document to be printed. In typical printing processes, multicolor images cannot be printed directly using a single printing plate. Rather, composite color images are first decomposed into a set of constituent color components, or "separations," each of which serve as the basis for an individual plate. The colors into which the multicolor image is decomposed depends on the particular "color model" chosen by the practitioner. The most common color model is based on cyan, magenta, yellow and black constituents, and is referred to as the "CMYK" color model. If the separation is performed properly, combination of the individual separations produces the original composite image.
Graphic-arts practitioners use proofing sheets (or simply "proofs") to correct separation images prior to producing final separation plates, as well as to evaluate the color quality that will be obtained during the printing process. A proof represents, and permits the practitioner to view, the final image as it will appear when printed.
A proof may be produced by irradiative or thermal transfer of a coloring agent, corresponding to one of the separation colors, onto a receiver sheet according to the distribution of that color in the final image. Transfer sheets corresponding to each color of the model can be applied to a single receiver sheet and sequentially imaged, producing a single-sheet proof of the final image. Alternatively, each of the color separations can be applied to a separate transparent receiver sheet, and the set of sheets superposed on each other in registration to reveal the final image.
Proofs can also allow the practitioner to preview the results of varying the separations or adding further elements to the composite. For example, one of the process colors can be emphasized to different degrees on separate transparent receiver sheets, and the different sheets alternatively combined with the remaining proofing sheets (representing the other process colors) to find the best combination. In other cases, the effect of adding of a further process or spot color (e.g., a metallic or a PANTONE color ) or a specialty application (e.g., spot lacquer, selectively applied varnish, etc.) to the standard color model can be examined by producing one or more additional transparent receiver sheets with the new color and overlaying this sheet on the four-color proof.
Because it is cumbersome to maintain registration among separate receiver sheets for the various colors, it is preferred to apply as many colors as possible to a single proofing sheet, viewing variations using transparent overlays. In the situation where a single process color is being varied, the remaining colors can be applied to a single-sheet proof and different transparent receiver sheets--each reflecting a different emphasis of the omitted color--overlaid thereon. Where the effect of adding colors beyond the standard model (or duplicating one of the colors for emphasis) is to be examined, a single-sheet proof of the image may be prepared and each additional color applied to a different transparent receiver sheet that may be overlaid onto the proof.
The single-sheet proof is most simply the receiver sheet itself, which has received some or all of the process colors. But printing is most typically performed on paper, which imparts its own visual impact; the appearance of a four-color image applied to a plastic receiver sheet will differ from the same image applied to paper. As a result, when comparing the effects of different printing variations, practitioners prefer to work with a paper proof against which the different transparencies are overlaid or laminated.
Conventional transfer materials are difficult to apply directly to paper. Consequently, a paper proof is typically prepared indirectly: first the separation colors are sequentially applied to a plastic receiver sheet to form the single-sheet proof described above, and the image of this proof is then transferred to paper by a heating process (such as lamination). Although this process produces a proof with substantial fidelity, the heating process shrinks both the receiver sheet and the paper substrate. The overlay, which has not undergone a heating process, cannot thereafter be registered against the paper proof with precision; its image has not undergone dimensional change, and is not the same size as the image on paper. Accordingly, there exists a need for a way to retain dimensional consistency between paper proofs and transparent image-bearing overlays created for use therewith.